Japanese Unexamined Patent Publication No. 9-509104 discloses a cutting insert 1b having a secondary sub cutting edge 5b parallel to a support surface 3, between a major cutting edge 4a and a sub cutting edge 5a, as shown in FIG. 14 in the publication.
However, when the cutting insert 1b is attached to a tool 14 with a negative axial rake angle, the secondary sub cutting edge 5b protrudes toward the front end of the tool 14 than the sub cutting edge 5a. There is therefore a risk of deterioration of the smoothness of a machined surface (bottom surface) of a workpiece.